using System;

public class Program

{

    public static void Main()

    {

        DateTime primerDiaMes = new DateTime(2021, 1, 1);

        for (int i = 0; i <= 1; i++)

                {                                

                    int addMonth = i == 0 ? 0 : 1;

                    int primerDiaMesValue = primerDiaMes.Month + addMonth;

            if(primerDiaMesValue > 12)

            {

                primerDiaMes = new DateTime(primerDiaMes.Year+1, 0 + addMonth, 1);

            }else{

                primerDiaMes = new DateTime(primerDiaMes.Year, primerDiaMes.Month + addMonth, 1);

            }

                    DateTime ultimoDiaMes = primerDiaMes.AddMonths(1).AddSeconds(-1);

            Console.WriteLine("primerDiaMes " + primerDiaMes);

        Console.WriteLine("ultimoDiaMes " + ultimoDiaMes);

                }

    }

}